Hi,
First of all, welcome to the community and thanks for your interest in coreboot!
Most DIP8 chips should work, but I would recommend at least 1MB(8MBit)
parts, so your payload would fit comfortably.
If you plan to use big payloads (anything involving a Linux kernel),
you should get 4MB or 8MB parts.
You can find flash parts on http://bios-repair.co.uk, but there are
many sites where you could find them, including ebay.
Cheers,
Cristi